Reverts To Default Theme – NOTHING WORKING! Aargh.

Since upgrading to 2.5 and installing a new theme for 2.5, my site reverts randomly to the default theme, though from the admin panel my custom theme seems to still be active.

Things that have NOT worked:
1. Forced upgrade of the database (appeared to work…next morning was back to default theme)
2. Revert to backup.
3. Re-name my theme as “default” (entire site was white)
4. Remove default so there’s nothing to revert to. (Only lasted until the next fatal error and went white when it tried to revert.)

Is there somewhere I can get an old version of wordpress to install to see if that works?

Any other suggestions? The only thing I can think of is editing the php call that tells it to revert to default, but I don’t know php!

Any suggestions would be much appreciated.
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